The present invention concerns press felts for use in the press section of papermaking machines. In the manufacture of paper products, a stock slurry consisting of about 1% papermaking fibers and others solids dispersed in about 99% water is delivered at high speed and precision from a headbox slice onto a rapidly moving forming fabric, or between two forming fabrics, in the forming section of a papermaking machine. The stock is subjected to agitation and is dewatered by various means through the forming fabrics, leaving behind a loosely cohesive and wet web of fibers. This web is then transferred to the press section where a further portion of water is removed by mechanical means as the web, supported by one or more press felts, passes through at least one, and usually a series, of press nips where water is essentially squeezed from the nascent sheet and into the press felt. The water is accepted by the press felt and, ideally, does not return to the web. The resulting sheet is then passed to the dryer section which includes a series of rotatable dryer drums, or cans, that are heated by steam. The sheet is directed around and held in contact with the periphery of these drums by one or more dryer fabrics so that the majority of the remaining water is removed by evaporation.
Press felts play a critical role in the manufacture of paper products. The known press felts are produced in a wide variety of styles designed to meet the requirements of the papermaking machines on which they are installed, and the paper grades being manufactured. They are generally assembled using a woven or nonwoven base fabric structure into which is needled one and usually multiple layers of a fibrous nonwoven batt. The batt provides a smooth surface upon which the paper product is conveyed, acts as a reservoir to trap water expressed at the press nip, and provides a measure of resiliency to the press felt as it passes through the nip. The base fabrics are typically woven from monofilament, cabled monofilament, multifilament or similar multicomponent yarns; they may also be arranged as nonwoven planar arrays. The component yarns are usually comprised of an extruded polymeric resin, typically a polyamide.
The base fabrics may be of single layer or multilayer construction, or they may be formed from two or more layers which are laminated together. They may be woven endless, so that the resulting fabric resembles a tube with no seam; such fabrics must be prepared to the length and width of the machine for which they are intended, and must be slipped onto the press section in a manner similar to a sock. An example of such a fabric is provided in U.S. Pat. No. 7,118,651. In a variant modified endless weaving technique, the weft yarns are used to form seaming loops at the widthwise fabric edges during manufacture; when installed on the papermaking machine, these yarns will be oriented in the intended machine direction (MD) allowing the fabric to be joined by bringing the loops from each side together and inserting a pin, or pintle, through the resulting channel formed by the intermeshed loops. An example of a modified endless woven fabric may be found in U.S. Pat. No. 3,815,645. The base fabrics may also be flat woven, using one or more layers of warp or weft yarns; a seam is typically formed at each end allowing the fabric to be joined on the machine. An example of a flat woven base fabric may be found in U.S. Pat. No. 7,892,402. All of the above constructions require that the base fabric be woven to the full width and length of the machine for which they are intended; this is a time-consuming process and requires high capital investment in wide industrial looms. In an effort to reduce manufacturing time and costs, so-called “multiaxial fabrics” have recently been introduced for the production of press felts.
Multiaxial press felts are well known and are described in U.S. Pat. No. 5,360,656; U.S. Pat. No. 5,268,076; U.S. Pat. No. 5,785,818 and others. The base fabrics of these press felts are comprised of a plurality of spirally wound and edgewise joined turns of a material strip including at least machine direction (MD) oriented yarns. The material strip is usually a flat woven fabric which is narrower than the width of the intended base fabric of which it is a component; it has also been proposed to use nonwoven arrays of MD yarns as the material strip component. Regardless of whether the component is woven or nonwoven, during assembly each turn of the material strip is directed about two opposing rollers such that its component MD yarns are canted at a small angle that is from about 1° to about 8° to the intended MD of the finished fabric; see prior art FIG. 1. Each successive turn of the material strip is edgewise bonded to that laid adjacent to it so as to build up a continuous tube-like base fabric of desired width and length. When removed from the assembly rollers and laid flat, the tube has continuous top and bottom surfaces joined at cross-machine direction (CD) oriented fold regions at each of the two opposing ends; see prior art FIG. 2. The completed multiaxial base fabrics are typically one of a two, three or four layer construction comprising the top and bottom surfaces of the spirally wound continuous tube, and optionally at least one additional flat fabric layer, located either interior to the flattened tube, or on top of one or both exterior surfaces. The assembled base fabrics may later be provided with a seam to facilitate their installation on the machine for which they are intended.
FIG. 3 shows the two opposing edge regions of the spirally wound prior art double layer woven structure of FIG. 2 with a portion of the CD oriented yarns removed at the opposing fold regions. This exposes the MD oriented yarns of the structure so that the yarn loops may be used to form a seam in the fabric as illustrated in FIG. 4. This Figure shows a double layer fabric that has been seamed by intermeshing the yarn loops formed by the MD yarns at the fold region and inserting a pintle across the length of the channel thus provided.
Regardless of its construction, the primary function of the press felt is to act as a reservoir to transport water expressed from the paper sheet as it passes through a press nip in the press section of the papermaking machine. The base fabric must therefore provide a measure of void volume, or empty interior space, into which the water can pass, and be held, until it can be removed at a later process stage. This space can be provided either by the weave structure of the base fabric in the manner described in U.S. Pat. No. 7,207,355 and as shown in cross-section in prior art FIG. 5, or the base fabric may include at least one additional fabric structure, such as a woven or nonwoven fabric, as mentioned above. Other constructions are possible.
Although spirally wound woven press felt base fabrics formed in the manner illustrated in FIGS. 1 to 5 have proven successful in many applications, they suffer from several disadvantages.
First, despite the fact that the component strips are narrow structures woven using a high speed loom, producing them is time consuming and costly. Weaving defects and other non-uniformities are often introduced during the production process because the weaving is often intermittent, requiring frequent process interruptions for feed-yarn resupply. These interruptions impart non-uniform areas into the woven material.
Second, the “knuckles” formed by the interwoven yarns in the component strip as they crimp about one another will become flattened due to the repeated passage of the fabric through the press nip(s), thus compacting the fabric. This compaction will affect dewatering performance over time as these fabrics typically have a “break-in” period during which they slowly adjust to their environment before they reach a steady state of performance. During this break-in period, their dewatering performance, void volume, permeability and other physical properties will change and it is generally necessary for the paper manufacturer to run the machine more slowly than would otherwise be desired and with frequent adjustment until a steady state of performance is reached. In addition, the interwoven yarns pass into and out of the surface planes in the weave structure, which ultimately reduces contact between the batt surface and paper sheet, and provides only a fraction of the yarn surface for planar sheet support. Such localized pressure points of the exposed crimped yarn knuckles can produce undesirable paper sheet marking. It will be apparent that such irregularities are undesirable as the base fabric and batt together should provide uniform planar support to the sheet for effective water removal and sheet smoothness.
Third, because the entire spirally wound fabric is woven according to the same weave pattern, interference patterns appear at locations where the warp and weft yarns from the opposing sides of the double layer tubular structure are not in exact alignment. Interference patterns are created because regions of relatively low and high yarn densities are formed as a result of the misalignment of the crimped yarns in the two opposing sides (i.e. they are not in exact registration with one another and move in and out of a horizontal plane as they nest between one another). If this situation is not addressed in some manner, the resulting press felt will also have areas of low and high yarn density which will be regularly spaced in both the length and width direction. This creates several problems, including: uneven water removal and sheet marking due in part to non-uniform batt adhesion. Batt fibers adhere more aggressively to base fabric regions where yarn densities are relatively high in comparison to areas where they are lower, resulting in fiber shedding where anchorage is comparatively poorer. This in turn will affect the uniformity of the fabric and thus its overall ability to evenly dewater the sheet without marking it.
Fourth, there are several seam related issues in the known seamed press felts. These include the lack of uniformity in the loops used to form the seam resulting in an uneven channel size, making the insertion of a pintle more difficult. Further, the physical characteristics in the seam area should, to the greatest extent possible, be the same as the remainder of the fabric. As the base fabric and any batt attached to it must usually be cut to install the seam, the physical properties of the seam region, including its surface uniformity, resistance to abrasive wear, and its overall dewatering and resiliency characteristics, are frequently the source of sheet marking or fabric failure. The seam region is thus usually recognized as the most critical area of the finished fabric.
During seam installation, at least a portion of the batt (and occasionally a portion of the component yarns of the woven or nonwoven base fabric) is cut to open the seam loop region and allow for removal of unwanted material adjacent the MD yarn loops. A “flap” of batt material is thus formed which must be securely reattached to the fabric so as to cover the seam region when the fabric is in use. This flap of material creates various problems in the finished press felt. As the batt flap begins to wear during use, some of the base fabric yarns at the cut edge may become loose and begin to pull out of the woven structure and batt, a phenomenon commonly known in the art as “stringing”. These exposed yarns will mark the sheet and promote more rapid degradation of the press felt at the seam region. In addition, because the base fabric is load bearing, this load may cause the base to retract back from the seam area, producing an open seam gap, which is also undesirable as it causes marking on the sheet.
Various efforts have been made to ensure secure batt anchorage where it is normally cut during seam installation and minimize discontinuities in the seam region. The solution most frequently used has been to insert so-called “stuffer yarns” into the base fabric adjacent the seam. These stuffer yarns are usually multicomponent yarns which, due to their larger surface area in comparison to monofilaments, offer greater opportunity for anchorage of the batt material during a needling process. Stuffer yarns have, in the past, been woven into the seam area on-loom (in full-width woven fabrics) allowing them to be located in a relatively fixed position. However, in multiaxial base fabrics, the stuffer yarns cannot be inserted during weaving and must instead be manually installed after the full width base fabric is assembled. Because both the MD and CD yarns in these fabrics are oriented at small angles of from 1° to 8° to the intended machine and cross-machine directions, the stuffer yarns are difficult to maintain in a constant position with the accuracy and permanency of those which are woven-in during weaving. As a result, the stuffer yarns tend to “wander” along the CD edge of the base fabric adjacent the seam loops, making the seam difficult to close during felt installation, reducing the effectiveness of batt anchorage at this area, and creating opportunity for discontinuity in the seam region.